To investigate the basis for widely disparate rates in the diagnosis of childhood hyperactivity between the U.S. and U.K., research teams and clinician panels in both countries rated a set of 36 standardized case histories, using both ICD-9 and DSM-III diagnostic schemes. Half of the cases had brief videotaped interviews. The rates for diagnosis of hyperactivity were affected by the diagnostic scheme, being considerably lower with ICD-9, which permits only one Axis I diagnosis. This effect was principally for British raters. With DSM-III, the rates were similar for British and American raters. Inter-rater agreement was acceptably high only for specially trained research teams: Within panels of experienced diagnosticians there was much disagreement. The presence of a videotape of part of the psychiatric interview did not affect inter-rater agreement.